Ceramic insulator



Patented Mar. 1 6, 1943 CERAMIC INSULATOR Lawrence Russel Shardlow, North Arlington, N. 1.. assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application December--27, 1939, Serial No. 311,108

7 Claims. (Cl. 106-62) My invention relates to ceramic objects, particularly to insulators for use in high voltage .high frequency fields, such as electrode spacers in electrode discharge devices.

An object of my invention is to provide a ceramic insulator which has good mechanical and insulating properties and which may be made in a wide range of firing temperatures.

Another object of my invention is to provide a ceramic insulator that has low power losses when used in a field of ultra high frequencies, even when operated at high temperatures.

The usual ceramics including the lavite and metallic oxides used extensively in radio tubes have a rather high power factor and dielectric constant, heat excessively in high frequency fields, and undesirable energy losses result. An electrode spacer with the usual factor in a radio tube operating at 70 megacycles, for example,

' large scale production the insulator should be easy to fire to the required hardness without vitrification within a wide range of tempera tures.

I have found that insulators with the necessary electrical characteristics for high frequency operation are preferably of small particles of metallic oxides, only the surfaces of the particles being fused together so that the particles are interlocked and the finished body is strong yet porous so that gases in the body may be removed.

My improved insulator consists of magnesium oxide or magnesia (MgO), silicon dioxide or silica (SiOz) and aluminum oxide or alumina.

(Al-203). I-make the bulk of my insulator body of finely ground particles of magnesia (MgO) and I propose according to the characteristic features of my invention to add binding materials to th body comprising respectively powdered acid magnesium meta-silicate, commercially known as talc, and an alkali-free clay of the kaolin variety. The magnesia may be fused or unfused and of commercial grade containing less than 0.2% alkali. When unfused magnesia is used it is preferably calcined for an hour at from 1500 to 1600 C. The magnesia is ballmilled until 90% of the material is finer than 2 microns in diameter and screened through a 100 mesh screen. The talc, commercially known as U. S. P. talc, and neutral to litmus is powdered and screened through a 325 mesh sieve, and the clay, preferably commercial kaolin free of impurities that cause dielectric losses such as iron oxides and alkali, is air floated to a particle size corresponding to a 325 or finer mesh sieve. One specific insulator with which good results maybe obtained is made by combining these three powders in the proportions by weight: 90% magnesia, 5% talc and 5% kaolin. A slip is prepared by adding 300 grams of the mixed powder to 250 cubic centimeters of carbon tetrachloride and 18 grams of organic binder material, such as domestic paraffin in a 1%, liter porcelain ball mill containing 1000 grams of flint pebbles. To insure complete and uniform coating of the particles of the aggregate with a film of paramn the slip should be ballmilled for several hours whereupon the slip may be poured from the ball mill and the carbon tetrachloride removed by slowly heating in air at about 110 for twelve hours. The resulting aggregate may be readily crumbled and screened, preferably through a mesh sieve. The resulting powder may now be pressed in steel molds into the desired insulator shapes by means of a plunger, green pieces that can be handled without breakage having been obtained by a hydraulic press applying a pressure of 5000 pounds a square inch to the ceramic.

I prefer to give the insulator bodies a preliminary firing in air for several hours by slowly raising the temperature to 220 C., where it is held for six hours, then to 800 C., where it is held for minutes, and then to 1100 C., where it is held for another 45 minutes. The insulators may then be cooled slowly and removed from the furnace. This preliminary firing removes the parafiin and renders the material strong and coherent enough to be drilled or out if desired.

The final firing and hardening of the insulator body is done in a hydrogen atmosphere at about 1450 C. for one hour, which produces a strong coherent insulator having a modulus of rupture sufiiciently high to meet all strength requirements of ordinary insulators, such as electrode spacers in radio tubes. The insulator is porous and has a uniform satiny white appearance as distinguished from the glassy vari-colored surface of vitrified ceramics. My insulator is especially efiicient at high frequencies as it has a power factor of only .03% at 400 C. ina megacycle field,

My improved insulator is adapted for large scale manufacture, the .necessary final firing temperature to produce a strong yet porous body is relatively low and is less critical than the usual metal oxide ceramics. Silicon dioxide aloneused as a binding agent has relatively poor bonding action for magnesia particles below about 1720". C. Kaolin, however, has a bonding action by itself at a temperature as low as 1200 C. The kaolin first absorbs and then reacts with the tused talc and forms a high viscosity glass which will not drain from the body at a temperature below about 1600 C. The high viscosity silicate glass is produced by the tale and kaolin over a wide range of temperatures and strongly bonds the magnesia particles.

While the body is conveniently prepared with the commercially obtainable powders of magnesia, talc and kaolin, the body of course could be prepared by admixing with the magnesia the metal oxides of the tale and kaolin and by adiusting the firing schedule. Pure kaolinite, free of alkalis and other impurities, essentially comprises about 40% A1202 compounded with 46% SiOz, the remainder being water; and talc comprises about 32% MgO and 64% silica, the remainder being water. The proportions of silica and alumina in kaolin found in nature may vary considerably, and in natural talc free silica is often present in addition to the silica compounded with magnesia. The proportions of the metal oxides quantitatively analyzed in my improved insulator will accordingly depend upon the particular clay and talc used in making the insulator.

While commercial powders in the proportions 90% magnesia, talc and 5% kaolin produce good insulators, the percent of these constituents may be varied over reasonably wide ranges. More than about 96.5% fused magnesia in the final body is friable and cracks easily, while the body containing less than 75.4% magnesia has a lower power factor and causes undesired power losses in ultra high frequency fields. Although equal parts of kaolin and talc are preferred, for one part of talc the kaolin may be varied between .43 and 2.33 parts. Larger proportions of talc apparently reduce the viscosity of the fiux at the final firing temperature so that the fiux drains from the body and hen the kaolin content is higher than 2.33 parts of kaolin to 1 part of talc, the bond is mechanically weaker, resulting in a mechanically poorer insulator. Powders added to the slip before firing in the proportions '10 to 95% magnesia, 1.5 to 21% talc and 1.5 to 21% kaolin will, after firing, be found by chemical analysis to contain magnesium 45.4% to 58.2%, silicon .12% to 8.4% and aluminum 32% to 4.55% corresponding to Per cent For each part of A1203 after firing, the S102 is kept within the limits of 1.87 to 4.95 parts silica. Variations in these percentages found by chemical analysis will be due to varying amounts of impurities.

The electrical insulators prepared by the ingradients in the proportions above specified produce an insulating body that is mechanically strong, that has low linear shrinkage during firing, that has extraordinarily high electrical insulating properties, and that may be fired in a relatively low yet wide range of firing temperatures. Since the good electrical and mechanical characteristics of my insulator may be obtained for different mixtures by the proper adjustment of firing schedule to obtain the partial fusion between the particles in the insulator body, it is desired that the proportions of constituents defined in the appended claims be broadly interpreted.

I claim:

1. An unvitrified ceramic body of magnesia, silica and alumina intimately admixed and fired into a strong coherent porous mass consisting of by weight 75.4 to 96.5% magnesia, 2.6% to 18% silica and .6% to 8.26% alumina, the magnesia comprising discrete particles bonded by glass containing said alumina and silica.

2. A ceramic electrical insulating body comprising by weight to 95% powdered magnesia, 1.5 to 21% talc and the remainder aluminum silicate, the body being substantially free of inorganic impurities that reduce power factor, the particles of magnesia being bonded together with an aluminum magnesium silicate.

3. A ceramic batch free of alkali iron oxides and impurities that increase high frequency electrical losses composed of about magnesia, about 5% kaolin and about 5% tale.

4. A coherent unvitrified porous ceramic insulator of which 76% to by weight is magnesia and the remainder is magnesium-aluminum silicate having for one part of alumina from about two parts to about five parts of silica, the magnesia particles being bonded by said silicate.

5. A porous coherent electrical insulator having low loss characteristics in a high frequency electrical field and consisting predominantly of particles of magnesia bonded by magnesiumaluminum silicate glass containing by weight in the insulator between 2.6 and 18% silica and .6 to 8.6% alumina, said insulator being substantially free of alkalis, iron, and impurities that reduce power factor.

6. A porous coherent ceramic insulator composed predominantly of magnesia particles, said particles being bonded by a glass consisting of approximately equal parts of aluminum silicate and of acid magnesium silicate, said insulator being substantially free of alkalis and of iron oxides and having a power factor about .03% at 400 C. in a '70 megacycle field.

7. A porous coherent ceramic electrical insulator having a power factor of about .03% at 400 C. in a high frequency field of 70 megacycles and composed of discrete particles of magnesia constituting at least 70% by'weight of the insulator, and of magnesium-aluminum silicate substantially free of iron, the alumina of the silicate constituting at least .6 by weight of the body and at least about one-fifth of the silica of the sillcate.

LAWRENCE RUSSEL SHARDLOW. 

